A traditional radio frequency (RF) transmitter (TX) architecture is shown in FIG. 1 which can be used to transmit a frequency or phase modulated signal. When designing an RF transmitter, the transmitter designer needs to deal with a troubling trade-off between the modulation bandwidth (BW) and the TX loop BW. To avoid any distortion in the modulated signal, the TX loop BW has to be designed to be much larger than the modulation BW. This requires a transmitter architecture having a large comparison frequency for the TX loop in order to minimize the spurious problem linked to the large bandwidth of the TX loop, as well as the critical noise response of the TX loop for the wide band noise (inside the TX loop BW itself).
In U.S. Pat. No. 5,953,895 a solution to the above noted problem is provided since in the approach described, the TX loop bandwidth and the modulation bandwidth are not related. The above noted patent attempts to solve the problem of distortion caused by the transmission PLL on the modulation spectrum with a method referred to as two-point modulation. The approach described however suffers from a major drawback presented by the critical noise response caused by the additional modulation added after the loop filter (item 403 in FIG. 1). In fact, the adding of an extra signal (in this case just part of the modulation) just in front of the VCO and after the loop filter, adds extra noise that is not be filtered out by the TX loop.
The TX loop in a transmitter acts as a frequency translator and as a low pass filter for the modulating signal. To avoid any signal corruption it is important to keep the TX loop BW larger than the modulation BW. In addition, to be compliant with present day modulation standards, a very low noise PLL is required, making quite difficult its design and integration. A need thus exist for a modulation scheme that allows for narrowing of the TX loop BW to relax the spec of the TX loop as well as to allow for easier integration. Such a modulation scheme should provide for a stable and low noise modulator where the modulation bandwidth is uncorrelated to the TX loop bandwidth.